operator-precedence

问题 What do you think would be the result of the next expression in Ruby? a = 10 && b = 25 Try to calculate in the ming and only then use irb . So, if we take a look at the Ruby documentation about Operators Precedence then we will se that && operator has a higher priority than = . So you must think that Ruby will evaluate the expression in the next way: a = ((10 && b) = 25) But Ruby does a job in another way: a = (10 && (b = 25)) # => 25 So, the priority of the = in b = 25 is higher, then && .

问题 Is it not possible to dereference a pointer to an object that's stored in an array using the indirection(dereference) operator or am I doing something wrong? #include <iostream> class A { public: virtual void test() { std::cout << "A\n"; } }; class B : public A { public: void test() { std::cout << "B\n"; } }; int main() { A* v[2]; v[0] = new A(); v[1] = new B(); v[0]->test(); *(v[1]).test(); // Error! If the arrow operator is used instead // though, the code compiles without a problem. return

问题 When I googled this I always got threads about order of evaluation in general stating order of evaluation is unspecified. I know the parameter evaluation order is unspecified in C in general. My question is parameter evaluation order in gcc , left to right or right to left ? Any links to resources would also be appreciated... EDIT: Removing ambiguity in the question Well, I'm talking about the situation when foo(1+2,2+3,4+9) which is first evaluated? is it 1+2 or 4+9 ... like wise.. Can we

问题 When I googled this I always got threads about order of evaluation in general stating order of evaluation is unspecified. I know the parameter evaluation order is unspecified in C in general. My question is parameter evaluation order in gcc , left to right or right to left ? Any links to resources would also be appreciated... EDIT: Removing ambiguity in the question Well, I'm talking about the situation when foo(1+2,2+3,4+9) which is first evaluated? is it 1+2 or 4+9 ... like wise.. Can we

问题 Found an interesting piece of code in Symfony core if ('' !== $host = $route->getHost()) { ... } The precedence of !== is higher than the = but how does it work logically? The first part is clear but the rest? I've created a little sample but it's still not clear: sample 回答1: The point is: The left hand side of an assignment has to be an variable! The only possible way to achieve this in your example is to evaluate the assignment first - which is what php actually does. Adding parenthesis

问题 I recently ran across the following situation: #include <iostream> int *p = 0; int f() { p = new int(10); return 0; } void g(int x, int *y = p) { std::cout << y << std::endl; } int main() { g(f()); } This is quite subtle, since you usually don't expect the default arguments to change during their evaluation for the function call. I had to take a look at the assembly to spot this error. Now my question is: Is this really undefined behavior, since there aren't any guarantees concerning the

问题 std::cout << (true ? "high pass" : false ? "fail" : "pass") is the same as std::cout << (true ? "high pass" : (false ? "fail" : "pass")) Since the ternary operator is right associative, why don't we perform the right-hand operation first? Shouldn't pass be printed instead of high pass ? 回答1: You misunderstood operator associativity. It's simply the way to group operators with the same precedence and doesn't affect order of evaluation in any way. So cond1 ? 1 : cond2 ? 2 : cond3 ? 3 : 4 will

问题 I'm translating some Fortran into Javascript and the order of operations for exponents is pretty opaque to me for a particular class of equations. Here's an example of the Fortran equation: x = 1+a*b**c*c**d Exponents in Fortran are designated with the ** operator. This page gives some hints: Arithmetic expressions are evaluated in accordance with the following priority rules: All exponentiations are performed first; consecutive exponentiations are performed from right to left. All

问题 When I study recursive functions in C from deitel c, I read this sentence: Standard C does not specify the order in which the operands of most operators (including +) are to be evaluated. But also the book says that: associativity of '+' from left to right Order of evaluation of operands: Could anyone explain why this is? 回答1: Order of evaluation and associativity are two different things, take the example: int x = func1() - func2() - func3(); //having int return types In this expression you

问题 In many of the answers that I found here were said the following words: Global variables in a single translation unit (source file) are initialized in the order in which they are defined. or Within the same compilation unit the order is well defined: The same order as definition. etc. But where can I see these words in the standard of C++? I would like to get a one or few concrete paragraph's where such behavior is described. I can not find it myself, and I do not know who to ask. 回答1: 6.6.3